Frequency synthesizer system and method

ABSTRACT

A frequency synthesizing circuit comprising a first mixer configured to receive a first input signal at a first input thereof, a first filter configured to receive an output signal of the first mixer and remove undesired signal frequencies from the output signal of the first mixer, and a feedback loop. The feedback loop includes a second mixer having a first input connected to the output of the first filter and a second input for receiving a second input signal. The second mixer is configured to mix a signal received at the first input with the second input signal. The feedback loop further includes a third mixer having a first input connected to an output of the second mixer and a second input for receiving a third input signal. The third mixer is configured to mix a signal received at the first input with the third input signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 14/034,087, now issued U.S. Pat. No. 9,112,483,entitled OFFSET REGENERATIVE FREQUENCY DIVIDER, filed Sep. 23, 2013, theentire contents of which is herein incorporated by reference for allpurposes.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under ContractN00024-00-C-5139 awarded by the Department of the Navy. The Governmenthas certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to frequency dividers, and morespecifically, to regenerative frequency dividers.

BACKGROUND

In various electronic applications, such as radar and communicationssystems, there is a need to synthesize or generate radio frequency (RF)signals across a wide range of frequencies. Many existing frequencysynthesizing techniques, however, are limited by the range of obtainableoutput frequencies, and/or by excessive noise introduced into theresulting output signals. For example, in analog and digital signalprocessing operations, frequency dividers are often implemented togenerate an output signal of a frequency f_(out), from an input signalof a frequency f_(in), where f_(out)=f_(in)/n, wherein “n” is aninteger. Likewise, in a traditional regenerative frequency divider, aninput signal is mixed with the output of the circuit via a feedbackloop. These dividers may achieve very low noise, but are limited tooutputs of one-half (½) the input signal frequency (or 1½ times theinput frequency). In order to obtain frequency divisions greater thantwo, multiple frequency dividers may be utilized. For example, these maybe arranged in cascade, but with increased complexity, cost, and noiseproduction.

Many applications, however, require production of low-noise frequenciesthat are not integer multiples of a given input frequency. In theseinstances, conjugate regenerative dividers, for example, are able toproduce fractional division but are complicated to stabilize. Likewise,complementary metal-oxide-semiconductor (CMOS) based dividers producelow noise, but are limited to input frequencies of about 200 MHz.Injection-locked frequency dividers have also been demonstrated at lownoise, but have known stability issues with high order divisions.

Alternative systems and methods for providing low-noise frequencygeneration at fractional division ratios are desired.

SUMMARY

In one embodiment of the present invention, a frequency synthesizingcircuit is provided. The circuit comprises a first mixer configured toreceive a first input signal at a first input thereof, a first filterconfigured to receive an output signal of the first mixer and to removeundesired signal frequencies from the output signal of the first mixer,and a feedback loop having an input connected to an output of the firstfilter and an output connected to a second input of the first mixer. Thefeedback loop includes a second mixer having a first input connected tothe output of the first filter and a second input for receiving a secondinput signal. The second mixer is configured to mix a signal received atthe first input with the second input signal. The feedback loop furtherincludes a third mixer having a first input connected to an output ofthe second mixer and a second input for receiving a third input signal.The third mixer is configured to mix a signal received at the firstinput with the third input signal. In one embodiment, the frequency ofthe first input signal is distinct from the frequency of the secondinput signal.

In another embodiment of the present disclosure, a frequencysynthesizing method is provided. The method includes the steps ofgenerating a first mixed signal by mixing a first input signal with anoutput of a feedback loop of a regenerative frequency divider. The firstmixed signal is filtered for removing undesired frequency signals. Asecond mixed signal is generated by mixing a signal derived from thefiltered first mixed signal with a second input signal within thefeedback loop. The second mixed signal is filtered for removingundesired frequency signals. A third mixed signal is generated by mixinga signal derived from the filtered second mixed signal with a thirdinput signal within the feedback loop. The output of the feedback loopis generated by filtering the third mixed signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a regenerative frequency divideraccording to the prior art.

FIG. 2 is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure, including anoffset signal introduced into the feedback loop of the divider.

FIG. 3 is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure, including twooffset signals introduced into the feedback loop of the divider.

FIG. 4 is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure, including afrequency divider and an offset signal introduced into the feedback loopof the divider.

FIG. 5A is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure.

FIG. 5B is a simplified diagram of a surface acoustic wave (SAW)oscillator utilizing the regenerative frequency divider of FIG. 5A.

FIG. 6 is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure.

FIG. 7 is a graphical illustration of the phase noise produced by theregenerative frequency divider of FIG. 5A.

FIG. 8 is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure.

FIG. 9 is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure.

FIG. 10 is a simplified diagram of an oscillating circuit useful forproducing the input signals provided to the regenerative frequencydivider of FIG. 9.

FIG. 11 is a simplified diagram of a regenerative frequency divideraccording to an embodiment of the present disclosure.

FIG. 12 is a process flow diagram illustrating a frequency translationmethod according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, many other elements found in frequencytranslation devices, including regenerative frequency dividers. However,because such elements are well known in the art, and because they do notfacilitate a better understanding of the present invention, a discussionof such elements is not provided herein. The disclosure herein isdirected to all such variations and modifications known to those skilledin the art.

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. It is to beunderstood that the various embodiments of the invention, althoughdifferent, are not necessarily mutually exclusive. Furthermore, aparticular feature, structure, or characteristic described herein inconnection with one embodiment may be implemented within otherembodiments without departing from the scope of the invention. Inaddition, it is to be understood that the location or arrangement ofindividual elements within each disclosed embodiment may be modifiedwithout departing from the scope of the invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims, appropriately interpreted, along with the full range ofequivalents to which the claims are entitled. In the drawings, likenumerals refer to the same or similar functionality throughout severalviews.

A regenerative frequency divider according to the prior art is shown inFIG. 1. Divider 10 includes a primary signal path 12 and a signalfeedback path 14. In operation, an input signal of frequency F_(in) issupplied to a frequency mixer 16 (e.g. a diode ring mixer), along withthe feedback signal from the output of divider 10 via feedback path 14.In the illustrated divider, the resulting output of mixer 16 are the sumand difference frequencies of ½ F_(in) and 1½ F_(in). Depending on thedesired output (in this case, F_(out)=½ F_(in)), a frequency blocking orremoval device, such as a filter 18 (e.g. a low-pass filter) is providedfor removing the higher frequency signal components. The resultingsignal of frequency ½ F_(in) output from filter 18 is then amplified viaamplifier 19, and output by divider 10. As set forth above, these typesof regenerative frequency dividers can be used to generate outputfrequencies having an integer relationship to a given input frequency(i.e. a “divide-by-two” relationship).

However, the ability to produce low-noise frequency translations beyondfrequencies having these integer relationships to the input frequency isroutinely required, despite the difficulties in achieving high qualityoutputs. Embodiments of the present disclosure provide a solution byintroducing frequency translation devices inside feedback loops ofregenerative dividers, providing the ability to generate variouslow-noise, fractional division ratios.

In one embodiment of the present disclosure, a frequency synthesizingcircuit is provided. The circuit comprises a first mixer for receiving afirst input signal, and a first removal or blocking device (e.g. afilter) configured to receive an output of the first mixer and to removeundesired frequencies from the output signal of the first mixer. Thecircuit further comprises a feedback loop for providing an output of thefirst filter to an input of the first mixer, wherein the first inputsignal and the output of the first filter are mixed. The feedback loopincludes a second mixer arranged therein and configured to receive theoutput of the first filter, and to mix the output of the first filterwith a second input or offset signal.

In another embodiment of the present invention, a frequency synthesizingmethod is provided. The method includes the steps of mixing a firstinput signal with an output of a feedback loop of a regenerativefrequency divider to provide a mixed signal that is then filtered toremove or attenuate signal components of undesired frequencies. A secondmixing operation is performed within the feedback loop using the outputsignal of the first mixing operation and a second input or offsetsignal. An output of this second mixing operation may be filtered priorto being input to the first mixing operation.

Regenerative frequency dividers according to embodiments of the presentdisclosure may take on numerous forms, and include various types offrequency translation devices (or combinations thereof) integrated intotheir feedback loops for performing a variety of operations. Referringgenerally to FIGS. 2 and 3, these operations may include mixing one ormore offset signals within the feedback loop, as well as performingmultiplication and/or division operations within the feedback loop (notshown). The offset signal or signals may be of the same or differentfrequencies compared to that of the input signal to the device.

Referring generally to FIG. 2, an offset regenerative divider accordingto an embodiment of the present disclosure is shown, which includes amixing function arranged within the feedback loop of the circuit.Divider 20 includes a first input signal of frequency F_(in) provided toa first mixer 21. A portion of the output of first mixer 21 is removedor blocked via a filter 22, which may comprise band-pass, high-pass, orlow-pass properties depending on the desired output frequency. Thefiltered output of first mixer 21 of frequency of F_(out1) may then beamplified by an amplifier 23. A second mixer 24 is provided in thefeedback loop of divider 20, and receives the output of divider 20(F_(out1)), as well as a second input or offset signal of desired offsetfrequency F_(offset). In one embodiment, F_(in) is a frequency whichdoes not have an integer relationship to (i.e. is not evenly divisibleby) frequency F_(offset) of the first offset signal input to secondmixer 24. Second mixer 24 may output through a second filter 25 (e.g.band-pass, high-pass, or low-pass filter) for passing frequency F₂ to asecond amplifier 26, the output of which is provided as a second inputto first mixer 21. The following formulas describe the relationshipsbetween the input, output and offset frequencies, assuming first andsecond filters 22,25 (FIG. 2) are configured as low-pass filters forpassing frequencies F₁ and F₂:F ₁ =F _(in) −F ₂  Eq. 1F ₂ =F ₁ −F _(offset)  Eq. 2

$\begin{matrix}{F_{1} = \frac{F_{in} + F_{offset}}{2}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

Referring generally to FIG. 3, a dual frequency offset divider accordingto an embodiment of the present disclosure is shown. Divider 30comprises features similar to those set forth above with respect to FIG.2, including an input signal of frequency F_(in1) provided to a firstmixer 31, a first filter 32 for passing frequency F₁, and a firstamplifier 33. The feedback loop comprises a first mixing stage,including a second mixer 34 responsive to the output of amplifier 33 aswell as a second input or offset signal having an offset frequencyF_(in2). A second filter 35 passes frequency F₂ to a second amplifier36. In this embodiment, the feedback loop further includes a secondmixing stage, including a third mixer 37 responsive to a third input oroffset signal of frequency F_(in3), as well as the output of secondamplifier 36. A third filter 38 is provided for passing frequency F₃ toa third amplifier 39 which outputs to first mixer 31. This dual mixingarrangement is represented as follows:F ₁ =F _(in1) −F ₃  Eq. 4F ₂ =F ₁ −F _(in2)  Eq. 5F ₃ =F ₂ −F _(in3)  Eq. 6

$\begin{matrix}{F_{1} = \frac{F_{{in}\; 1} + F_{{in}\; 2} + F_{{in}\; 3}}{2}} & {{Eq}.\mspace{14mu} 7}\end{matrix}$

FIG. 4 illustrates a hybrid regenerative frequency divider arrangementaccording to an embodiment of the present disclosure, whereinmultiplication and/or division operations are performed within thefeedback loop of the divider, in conjunction with the above-describedoffset frequency mixing operations. In the illustrated exemplaryembodiment, divider 60 comprises a first mixer 61, a first filter 62,and a first amplifier 63 for outputting a signal having frequency F₁. Afrequency divider 64 is provided in the feedback loop of divider 60, theoutput signal (of frequency F₁/N) of which is provided to a secondfilter 65 for passing signal frequency F₂ to a second amplifier 66. Asecond mixer 67 is responsive to a second input or offset signal havingfrequency F_(offset), as well as the output of second amplifier 66.Mixer 67 outputs to a third filter 68 for passing resulting frequency F₃to a third amplifier 69 used to generate the second input to first mixer61, wherein:F ₁ =F _(in) −F ₃  Eq. 8

$\begin{matrix}{F_{2} = \frac{F_{1}}{N}} & {{Eq}.\mspace{14mu} 9}\end{matrix}$F ₃ =F ₂ −F _(offset)  Eq. 10

$\begin{matrix}{F_{1} = \frac{F_{in} + F_{offset}}{1 + \frac{1}{N}}} & {{Eq}.\mspace{14mu} 11}\end{matrix}$

FIGS. 5A through 11 provide exemplary application-specific embodimentsof dividers according to the present disclosure. FIG. 5A is a simplifieddiagram of a regenerative frequency divider 70 used to convert a 640 MHzinput signal to a 400 MHz output signal according to an embodiment ofthe present disclosure. The embodiment includes a first mixer 72, afirst filter 73, and a first amplifier 74. A second mixer 76 is arrangedin the feedback loop of divider 70, along with a second filter 77, and asecond amplifier 78. A frequency divider 79 is used to generate a signalhaving an offset frequency of one-quarter (%) of the input frequency(e.g. 160 MHz) for providing to second mixer 76. The output of secondmixer 76 comprises signal components at both 560 MHz and 240 MHz.Low-pass second filter 77 is used to pass the 240 MHz signal to an inputof first mixer 72 for generation of the 400 MHz output of the divider.FIG. 5B is a simplified diagram of a surface acoustic wave (SAW)oscillator 71 utilizing the regenerative frequency divider 70 of FIG. 5Ain a phased-lock loop arrangement. FIG. 7 is a graphical illustration ofthe phase noise 90 produced by the regenerative frequency divider ofFIG. 5A.

FIG. 6 illustrates another embodiment of the present disclosure, whereina frequency divider 81 is used in the feedback loop of a regenerativedivider 80 to generate a 320 MHz output signal from a 400 MHz inputsignal.

Other embodiments of the present disclosure include regenerativedividers utilizing; 400 MHz and 200 MHz (offset) inputs for generating a640 MHz output (FIG. 8, divider 100), and 3200 MHz and 1600 MHz (offset)inputs for generating a 5120 MHz output (FIG. 9, divider 110). FIG. 10is a simplified diagram of an oscillating circuit 120 useful forproducing the input signal to regenerative frequency divider 110 of FIG.9. Specifically, a plurality of frequency multipliers 121 are used toproduce the 1600 MHz and 3200 MHz input signals to divider 110.

FIG. 11 is a simplified diagram of a hybrid regenerative frequencydivider 130 according to an embodiment of the present disclosure. In theillustrated embodiment, divider 130 includes an input signal offrequency F_(in) provided to a first mixer 131. Two filters 132,133 arearranged in parallel and responsive to the output of first mixer 131;filter 132 providing the final output of divider 130, and filter 133used to generate a desired offset signal to mixer 131. Morespecifically, the output of filter 133 is amplified via amplifier 134,and provided as an input to a second mixer 136. A frequency divider 135is responsive to the input signal (F_(in)) for providing the secondinput to second mixer 136. A third low-pass filter 137 passes the signalof frequency 3F_(in)/8 to a second amplifier 139. A second frequencytranslation device 138 (e.g. frequency divider such as a regenerativefrequency divider) is also responsive to the modified (i.e. halved)input signal (F_(in)/2) to produce an output of 3F_(in)/4. This output,along with the output of second amplifier 139 is input to a third mixer140, low-pass filtered by filter 141, amplified at amplifier 142, andprovided as the second input to first mixer 131. The result is a broadband frequency translation device capable of generating, for example,500 MHz to 4.5 GHz outputs, in 50 MHz steps, from a 4 GHz input, whereineach mixer input signal (i.e. six input signals) is derived from a firstinput signal to the divider.

FIG. 12 illustrates an exemplary frequency translation method 150according to an embodiment of the present disclosure. In step 152 afirst mixing operation includes mixing a first input signal with anoutput of a feedback loop of a regenerative frequency divider togenerate a first mixed signal. Step 154 includes filtering (e.g.low-pass filtering) the first mixed signal. A second mixing operation isperformed within the feedback loop which includes mixing the filteredfirst mixed signal with a second input signal (step 156). In step 158,the resulting mixed signal from the second mixing operation is filtered,and subsequently input to the first mixing operation in step 160.

While the foregoing invention has been described with reference to theabove-described embodiment, various additional modifications and changescan be made without departing from the spirit of the invention.Accordingly, all such modifications and changes are considered to bewithin the scope of the appended claims. Accordingly, the specificationand the drawings are to be regarded in an illustrative rather than arestrictive sense. The accompanying drawings that form a part hereof,show by way of illustration, and not of limitation, specific embodimentsin which the subject matter may be practiced. The embodimentsillustrated are described in sufficient detail to enable those skilledin the art to practice the teachings disclosed herein. Other embodimentsmay be utilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations of variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

What is claimed is:
 1. A frequency synthesizing circuit comprising: afirst mixer configured to receive a first input signal at a first inputthereof; a first filter configured to receive an output signal of thefirst mixer, remove undesired signal frequencies from the output signalof the first mixer, and output a signal of a first filtered frequency;and a feedback loop having an input connected to an output of the firstfilter, and an output connected to a second input of the first mixer,the feedback loop comprising: a second mixer having a first inputconnected to the output of the first filter, and a second input forreceiving a second input signal, the second mixer configured to mix thesignal of the first filtered frequency received at the first input withthe second input signal; and a third mixer having a first inputconnected to an output of the second mixer, and a second input forreceiving a third input signal, the third mixer configured to mix asignal received at the first input with the third input signal.
 2. Thefrequency synthesizing circuit of claim 1, wherein the frequency of thesignal received at the first input of the second mixer is dependent onthe frequency of the output of the first filter.
 3. The frequencysynthesizing circuit of claim 1, wherein an output of the third mixer isprovided to the second input of the first mixer.
 4. The frequencysynthesizing circuit of claim 1, wherein the frequency of the firstinput signal is distinct from the frequency of the second input signal.5. The frequency synthesizing circuit of claim 1, further comprising asecond filter configured to receive an output of the second mixer,remove undesired signal frequencies from the output signal of the secondmixer, and output a signal of a second filtered frequency.
 6. Thefrequency synthesizing circuit of claim 5, wherein the third mixer isconfigured to mix the signal of the second filtered frequency receivedat the first input with the third input signal.
 7. The frequencysynthesizing circuit of claim 1, wherein the feedback loop furthercomprises a frequency divider arranged between the output of the firstfilter and the first input of the second mixer.
 8. The frequencysynthesizing circuit of claim 7, wherein the feedback loop furthercomprises a second filter arranged between an output of the frequencydivider and the first input of the second mixer.
 9. The frequencysynthesizing circuit of claim 1, wherein the third input signal isderived from the first input signal.
 10. A frequency synthesizingcircuit comprising: a first mixer configured to receive a first inputsignal at a first input thereof; a first filter configured to receive anoutput signal of the first mixer and remove undesired signal frequenciesfrom the output signal of the first mixer; and a feedback loop having aninput connected to an output of the first filter, and an outputconnected to a second input of the first mixer, the feedback loopcomprising: a second mixer having a first input connected to the outputof the first filter, and a second input for receiving a second inputsignal, the second mixer configured to mix a signal received at thefirst input with the second input signal; and a third mixer having afirst input connected to an output of the second mixer, and a secondinput for receiving a third input signal, the third mixer configured tomix a signal received at the first input with the third input signal,wherein the second input signal is derived from the first input signal.11. The frequency synthesizing circuit of claim 10, wherein the thirdinput signal is derived from the first input signal.
 12. The frequencysynthesizing circuit of claim 11, further comprising at least one of afrequency multiplier and a frequency divider configured to receive thefirst input signal and output the second input signal.
 13. The frequencysynthesizing circuit of claim 12, further comprising a frequency dividerresponsive to the second input signal for generating the third inputsignal.
 14. The frequency synthesizing circuit of claim 13, wherein thefrequency divider comprises a regenerative frequency divider.
 15. Afrequency synthesizing method comprising the steps of: generating afirst mixed signal by mixing a first input signal with an output of afeedback loop of a regenerative frequency divider; filtering the firstmixed signal to remove undesired frequency signals and output a signalof a first filtered frequency; generating a second mixed signal bymixing the signal of the first filtered frequency with a second inputsignal within the feedback loop; filtering the second mixed signal forremoving undesired frequency signals; generating a third mixed signal bymixing a signal derived from the filtered second mixed signal with athird input signal within the feedback loop; and generating the outputsignal of the feedback loop by filtering the third mixed signal.
 16. Themethod of claim 15, wherein the second input signal is derived from thefirst input signal.
 17. The method of claim 16, further comprising atleast one of the steps of: a) multiplying, and b) dividing, the firstinput signal to produce the second input signal.
 18. A frequencysynthesizing method comprising the steps of: generating a first mixedsignal by mixing a first input signal with an output of a feedback loopof a regenerative frequency divider; filtering the first mixed signal toremove undesired frequency signals; generating a second mixed signal bymixing a signal derived from the filtered first mixed signal with asecond input signal within the feedback loop; filtering the second mixedsignal for removing undesired frequency signals; generating a thirdmixed signal by mixing a signal derived from the filtered second mixedsignal with a third input signal within the feedback loop; andgenerating the output signal of the feedback loop by filtering the thirdmixed signal, wherein the third input signal is derived from the firstinput signal.
 19. The method of claim 18, further comprising at leastone of the steps of: a) multiplying, and b) dividing, the first inputsignal to produce the third input signal.
 20. The method of claim 18,wherein the second input signal is derived from the first input signal.